1. Field of the Invention
This invention relates to a process for producing an O-acetyl compound such as acetic anhydride by reacting and O-methyl compound as a starting material, such as methyl acetate, with carbon monoxide by a catalytic reaction in which rhodium is used as a principal catalyst.
2. Description of the Prior Art
Acetic anhydride is used in quantity as a material for the production of cellulose acetate and, in addition, it is useful as a material for medicines, perfumes, dyes, etc.
Acetic anhydride has heretofore been produced industrially by a process in which ketene obtained by the thermal decomposition of acetic acid is reacted with acetic acid.
On the other hand, studies are being made actively to produce acetic anhydride by a reaction of carbon monoxide with methyl acetate or dimethyl ether. Although this reaction can proceed under a milder condition in a process in which rhodium is used as a principal catalyst than in processes in which other transition metal catalyst are used, it is necessary in order for this process to be actually used in the industry that a rhodium catalyst used in a carbonylation reaction can be recycled in an active form after it is separated from the desired product.
Japanese Patent Publication No. 2216/1983 (Japanese Patent Laid-Open No. 51036/1980) discloses a technique for preventing loss of the activity of a Group VIII noble metal catalyst in the separation step by improving the step of catalyst separation in the production of a carboxylic acid anhydride by carbonylation. This process comprises application of a partial pressure of hydrogen of at least 0.7 kg/cm.sup.2 when the reaction mixture obtained in the carbonylation is separated into a volatile component and a catalyst solution. According to this process, it is preferred to maintain the partial pressure of carbon monoxide at 1.05 kg/cm.sup.2 or above when a metallic promotor such as chromium is used, and consequently, as seen in the examples of this patent, a pressure-resistant vessel is necessary in the separation step (flash distillation), and it is probable that the compressed gas is carried away together with a volatile component.
A process is also known in which a considerable amount of hydrogen is incorporated in a reaction gas in the production of acetic anhydride from methyl acetate and carbon monoxide with the aid of a Group VIII noble metal catalyst (Japanese Patent Laid-Open No. 65709/1976). Although it is described that this process exerts an effect of preventing the formation of carbon dioxide, by-products such as ethylidene diacetate, acetaldehyde, acetone and methane, in addition to acetic acid mentioned in this publication, increase.